Multiphase polymer blends are of major economic importance in the polymer industry. In general, commercial polymer blends consist of two or more polymers. In some cases, they may be combined with small amounts of a compatibilizer or an interfacial agent.
Traditional and non-chemically modified polypropylenes have limited processability in applications that require melt strength. Without sufficient melt strength, polypropylene exhibits limited capability in conversion processes such as large part blow molding, thermoforming, extrusion coating, and some blown film applications. Chemically cross-linked or coupled polypropylenes can address some of these limitations, but crosslinked polymers generally have poorer mechanical properties (e.g. low extensibility). The additional cost of various cross-linking technologies such as azide-modification or E-beaming can also be cost prohibitive. It would be highly desirable to improve the melt strength of polypropylene by compatibilizing it with other polyolefins that have higher melt strength such as polyethylenes containing high levels of long chain branching such as Low Density Polyethylene (LDPE). However, blends of polypropylene and most polyethylenes are incompatible and result in immiscible blends with poor mechanical and optical properties.
Block copolymers can be used as compatibilizers. Block copolymers comprise sequences (“blocks”) of the same monomer unit, covalently bound to sequences of unlike type. The blocks can be connected in a variety of ways, such as A-B in diblock and A-B-A triblock structures, where A represents one block and B represents a different block. In a multi-block copolymer, A and B can be connected in a number of different ways and be repeated multiply. The block copolymer may further comprise additional blocks of different type. Multi-block copolymers can be either linear multi-block, multi-block star polymers (in which all blocks bond to the same atom or chemical moiety) or comb-like polymers where the B blocks are attached at one end to an A backbone.
A block copolymer is created when two or more polymer molecules of different chemical composition are covalently bonded to each other. While a wide variety of block copolymer architectures are possible, a number of block copolymers involve the covalent bonding of hard plastic blocks, which are substantially crystalline or glassy, to elastomeric blocks forming thermoplastic elastomers. Other block copolymers, such as rubber-rubber (elastomer-elastomer), glass-glass, and glass-crystalline block copolymers, are also possible.
One method to make block copolymers is to produce a “living polymer”. Unlike typical Ziegler-Natta polymerization processes, living polymerization processes involve only initiation and propagation steps and essentially lack chain terminating side reactions. This permits the synthesis of predetermined and well-controlled structures desired in a block copolymer. A polymer created in a “living” system can have a narrow or extremely narrow distribution of molecular weight and be essentially monodisperse (i.e., the polydispersity index (PDI) is essentially one). Living catalyst systems are characterized by an initiation rate which is on the order of or exceeds the propagation rate, and the absence of termination or transfer reactions. In addition, these catalyst systems are characterized by the presence of a single type of active site. To produce a high yield of block copolymer in a polymerization process, such catalysts must exhibit living characteristics to a substantial extent.
Another method for producing block copolymers involves the use of chain shuttling technology. Such methods are exemplified in, for example, WO2005/090425, WO2005/090426, WO2005/090427 and WO2007/035489. In chain shuttling, block copolymers may be produced by shuttling a growing polymer chain between two or more catalysts in a given reactor environment whereby each catalyst makes a type of polymer that is distinct in composition. The catalysts can make polymers which differ in the amount or type of comonomer incorporated therein, the density, the amount of crystallinity, the crystallite size attributable to a polymer of such composition, the type or degree of tacticity (isotactic or syndiotactic), regio-regularity or regio-irregularity, the amount of branching, including long chain branching or hyper-branching, the homogeneity, or any other chemical or physical property. The shuttling mechanism employs one or more shuttling agents, which do not make polymer, but serve to transfer the polymer between active catalyst sites. Alternatively, chain shuttling may be employed to produce a block copolymer by employing two or more reactors in series. In this case, the shuttling agent acts to extend the average life of a growing polymer chain such that the polymer chains experience chain growth in each reactor before termination. The composition of each of the polymer blocks is determined by the catalyst(s) and the reactor conditions.